Magnitude 8.9 (or 9.0, or 9.1!) Earthquake off the coast of Japan

A post by Chris RowanAround 3pm local time yesterday, there was a massive earthquake about 100 miles off the east coast of northern Honshu Island, Japan. Initially calculated to be a magnitude 8.9, it has since been upgraded: the current CMT solution at the USGS has it as a 9.1. Either way, this is the biggest instrumentally recorded earthquake Japan has ever been shaken by, and is one of the biggest ever detected: it’s up there with the 2004 Boxing Day earthquake, and like that earthquake it generated a large – and extremely damaging – tsunami. The footage from the Honshu coast almost defies belief.

There is already lots of excellent coverage of this earthquake from the geoblogosphere: Callan Bentley has an excellent summary of the characteristics of the earthquake and the tectonics that generated it, and over at Georneys Evelyn provides some excellent explanations of the geological forces acting at subduction boundaries, and Japan in particular. A comprehensive list of other posts on the earthquake is being compiled by Silver Fox over at Looking For Detachment.

Nonetheless, I think there’s still a few issues that I can try to clear up:

Which fault actually ruptured in this earthquake?

Japan is situated in a complicated plate boundary region where three subduction zones meet. This particular earthquake is on the part of the boundary where the Pacific plate is being subducted west beneath the North American plate (yes, really: the not-particularly active boundary between the North American plate and the Eurasian plate runs through Siberia, down the western edge of the Sea of Okhotsk and through Japan). Unsurprisingly, the focal mechanism indicates compression, along either a shallowly west-dipping or a steeply east-dipping fault (primer on focal mechanisms)

Focal mechanism for the main shock, and cross-sections of the two possible fault orientations

[Note: due to a severe foul up in my initial analysis, I've had to heavily modify this section. Contrary to what I originally wrote, the focal mechanism is entirely consistent with movement on the subduction interface, so is not a splay fault as I originally proposed. Many thanks to Kim Hannula and Eric Fielding, who both corrected me via e-mail. I am now heading to stereonet bootcamp]

This is consistent with motion on the subduction interface, and even though the rupture was 150 km behind the trench where the plate boundary intersects with the seafloor, it seems to have propagated most or all of the way to the surface, producing large, sudden vertical movement of the sea-bed and the overlying water and generating a tsunami.

The rupture appears to have propogated to the sea-floor, generating a tsunami.

What’s with the changing magnitude estimates?

To estimate earthquake magnitudes, you look at the amplitude of the seismic waves it generates: the larger the amplitude of the waves, the larger the magnitude of the earthquake that produced them. However, in very large earthquakes, this relationship starts to break down, at least for the frequencies of seismic waves that are generally used to produce the quick magnitude estimates: they ‘saturate’, or stop increasing in amplitude as the earthquake magnitude does. This means that the magnitude estimates for the largest earthquakes will be somewhat underestimated until seismologists look at lower frequency waves, which are less susceptible to this saturation effect.

Were there foreshocks earlier in the week, and did they warn a bigger quake was on the way?

Yes to the first, no to the second. On Wednesday, there was a magnitude 7.2 earthquake in the same region as today’s earthquake, with a very similar-looking compressional focal mechanism, which was followed by a number of smaller >M 5 quakes, including three M 6-6.1 events (the focal mechanisms for two of these are also shown in the figure below; there is no focal mechanism available for the third). These were mainly clustered in a region just to the northeast of today’s larger rupture, and within the much larger cloud of aftershocks that are still being produced by that event (at my last count, there have been more than 100 aftershocks of greater than magnitude 5, and almost 20 of greater than magnitude 6). Thus, in hindsight, these earthquakes were clearly foreshocks of today’s main event.

Map showing location of seismicity on 9th and 10th of March (yellow circles) compared to March 11's ~M9 (largest orange circle) and its aftershocks (other orange and red circles).

Focal mechanisms of three >M6 earthquakes on 9th Match, located close to today's rupture.

However, there was no way of telling this in advance: there is nothing particularly “foreshock-y” about foreshocks beyond the fact that they end up being smaller in magnitude than the main shock they precede. In fact, if you plot the last few days of earthquakes over time, you can see that, on Wednesday and Thursday, seismic activity seemed to be dying down again in the wake of Wednesday’s 7.2 quake.

Magnitude of earthquakes (M5-6=small yellow circles, M6-7 orange circles, M7+ large red circles) off the coast of Honshu, 9-11 March.

What didn’t cause this earthquake?

In case you were wondering, this earthquake was not anything to do with:

  • The moon (or the ‘Supermoon’, which is the woo-ified way of saying ‘the Moon, a teeny tiny bit closer to Earth than it is on average) – Erik Klemetti has a good dismantling of this one.
  • The Christchurch earthquake – devastating as it was due to it’s location, this month’s M 6.3 earthquake in Christchurch was not that large in the grand scheme of things, releasing around 30 times less energy even than Wednesday’s 7.2 foreshock.
  • Non-existent US government earthquake weapons
  • The run-up to 2012 (we’re still way behind the 1960s)

Sadly, devastating events like this are a part of the way the planet we live on works. Despite claims and pseudo-predictions to the contrary, they require no special explanation beyond normal plate tectonics, with all of the unpredictability – on human timescales, at least – that that implies.

Categories: earthquakes, focal mechanisms, geohazards, tectonics
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Comments (12)

  1. Don says:

    Thanks for the great time vs mag chart and the epicenter map, I was trying to get a picture. People in Japan have much more important things to worry about than explanations at this time. As always, when an event like this happens: Chile, Banda Aceh, Port-au-Prince, Sendai – Northgate, Loma Prieta, Anatolia, China or where-ever, I think of Cascadia, which is about 75 miles west of me, and wonder when…

    As you state, foreshocks are great in retrospect but pretty useless in prediction. I used to live near Parkfield, CA but moved shortly before the Paso Robles quake wrecked the old town. Even minor quakes can be destructive if conditions are right for it. A couple of people killed is only important if it’s someone you know or if it’s your town – or if it’s you.

    I hope I’m wrong but I fear that the death-toll from this Sendai quake will be far larger than anyone is talking of. The Japanese people are very strong and resilient and have resources – and resolve – to help those who survive the initial trauma. Which is not to say that we, all, need not try to help as humanitarians and friends.

    Still, I wonder if people remember the poor and battered people of Haiti – some of the least wealthy people on our world, and possessed of a ruling class which has little, if any, concern for the common people – many of whom are still clinging to life in hovels and tents after losing what little they had before the earthquake.

    I visited Port-au-Prince for a few days back when Papa Doc was still in charge and in many ways, the old city reminded me of Manila – that was in the early 60s, the same type of tropical colonial ambience although the languages and cultures were utterly unlike one another. The poverty, though, that was a lot alike. And still is…

  2. jean cave says:

    There is an interesting correlation between solar flare activity and size of earthquakes.
    I accept nature functions and there is no esoteric mystery with that, but we now have the means to collect and collate info on new stuff that we didn’t know about before, that does have a bearing. THEMIS data on sun activity and spacequakes in the magnetosphere for example has only become possible in the last few years. I am looking forward to a predictive model from this research.

    • Chris Rowan says:

      The phrase ‘Interesting correlation’ is rather meaningless in itself. Unless by ‘interesting’, you mean ‘none at all, which is hardly surprising given the physical implausibility of any connection between’

  3. bruce stout says:

    Dang it, I thought I had the beach balls under control but obviously not.

    I thought the lines on the balls represented the possible fault orientations with the red quarters showing the primary motion of the quake (with the maximum force shooting off at 45° to the fault plane) but your suggested fault orientations don’t agree with the lines on the ball. I must have got something wrong (in addition to being too simplistic ;-))

    • Chris Rowan says:

      The thing to remember (and I need to remember this too occasionally, it seems!) is that you’re looking at a lower hemisphere projection – the lines are projections of planes dipping into your screen rather than out of it.

      • bruce stout says:

        Ah! penny drops. So your suggested fault orientations are a side view and not the view from space (like the beach ball). Which is also why they agree with the schematics in the next graph down the page. doh! What a relief!

  4. anonymous says:

    I would call it underestimating the potential of foreshocks. I agree we dont have any empirical formula (someting like omori’s law) but still foreshock still have its importance. In the above diagram as we can noticing the foreshocks vanishing prior to the main shock are usually considered seismic quiescence period (in retrospection), which indicate the development of stress in the region. When stress accumulation crosses the threshold stress value (abstract term) the is a shock. Such quiescence periods have been noticed before many large earthquakes. During this time window there are drastic changes in b-value and z-value (which seismologist usually use).
    Basically there are two schools of seismologist, one believing that we can predict an earthquake to some extent and another who believes that it is impossible.

    Sometimes even radon gas anomaly is used for studying earthquake precursor.

    Regarding the recent volcano in Japan, it is could also have been triggered by the earthquake. The stress enhancement and the changes in fluid flow caused by quake are the prominent reason.

  5. pasionaria says:

    Hi everybody! I’m an italian geologist. I live in Venice. I’m writing in order to express my worry about our government’s plan to use nuclear power for the new energetic policy. Italy is one of the most seismic land in whole Europe and the earthquake happened in Abruzzo (2009) harmed either population and our art heritage..The earthquake in Japan was even stronger (x30000) – and therefore more harmful..I wonder what could have caused if such an earthquake happened here? Can new generation nuclear power station relief this worry?
    What do you think about?

    • Chris Rowan says:

      Well, although Italy is seismically active, I don’t think there’s any fault in the area large enough the produce a magnitude 9 earthquake. And remember, it was the tsunami that did the damage, and that damage was not to the reactor itself (which shut down as it was designed to), but the cooling systems around it.

      Which is not to say that building nuclear power stations in any seismically active zone is something that should be done lightly, of course.

      • pasionaria says:

        Our Mar Tirreno is deep enough to provoce very big tsunami; many active faults, volcanoes and platform slides can produce it. I think wrong localization of the Fukushima nuclear power station (not the earthquake itself: M=5) is the most important factor of the disaster…and Italian coast is 8000 km long..

  6. Darren Addy says:

    A commenter on another forum I’m on said the following:
    If the faults are converging at 1 inch per year, the displacement of this quake was 8 feet, and most fault energy is released in a few large earthquakes, then you would expect a similar quake from this particular fault every ~100 years. Now these assumptions are decent, but obviously not perfect, however I think they should at least get us in the right ballpark.

    This seems overly simplistic to me, both ignoring the fact that all of the quakes over the years were releasing bits of tension and also that the tension after the quake is not necessarily “zero”. Would you care to comment on his method and conclusion that something like this must happen every 100 years or so?

    • Chris Rowan says:

      The fault moved a bit more than 8 feet; those displacements were on the Japanese coast, some distance away from the rupture itself, which some estimates suggest moved up to 180 feet in places.

      Also, plate convergence at the Japan Trench is more like 3 inches (9 cm) a year than 1. So the math is a bit wrong. But you’re right in a more general sense – not all the stress actually has to be released in one go, especially if there are strong zones (asperities) on the megathrust.

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